Who would have guessed?

When the world’s most common insecticides, called neonicotinoids, came on the market in the late 1980s, they were intended to be safer for bees than alternatives. Studies eventually showed that they were actually harming bees—and a new study offers new evidence that the chemicals are also harming birds.

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“With neonicotinoids, the original thought was that they’re only harmful to insects, and they’re safe for mammals and birds,” says Bridget Stutchbury, a biology professor at York University. “But nobody had really done these kinds of dosing studies before on migratory songbirds.” In an earlier study, Stutchbury and other researchers tested what happened when birds ate the insecticides in the lab. “We found that even small doses cause them to get very sick,” she says. “We were alarmed and surprised, and thought, well, what happens to birds in the wild?”

In the new study, the researchers gave wild sparrows tiny doses of the chemicals—as much as they would be exposed to by eating only two or three tiny seeds coated in the insecticide. (A larger dose, of 10 or 15 seeds, would likely be lethal.) Then they tracked what happened when the birds were released back into the wild during their spring migration, using lightweight radio tags and a huge network of radio towers that recorded the birds’ locations. “We could piece together how long it took them to resume migrations after they had been dosed,” Stutchbury says.

When birds ingested the pesticide, they lost 6% of their body weight within six hours. They also stayed in place for an average of 3.5 days longer than they normally would have. “Being sick for a couple of days could expose them to risk of predators,” she says. For birds with a short breeding season, arriving late might also hurt their chances of having a family. In the lab study, the team observed an additional problem: when the birds got sick, they also became disoriented and couldn’t tell north from south, a critical skill for a migratory animal. In the wild, it’s likely that the birds waited to begin flying again only after they were able to navigate; they did eventually fly in the correct direction.

When the insecticides are coated on seeds, farmers have often argued that birds won’t be exposed because the seeds are planted underground. But other research has shown that it’s common for seeds to spill during the planting process. Birds may also be exposed to the chemicals in other ways. New noninvasive blood tests can confirm how much of the pesticide they have in their system. “You can run the blood sample and find out how much is in the bird’s system,” says Sutchbury. “So you don’t have to argue about whether they finding seeds or not.”

For wild birds, there are many reasons that populations are dropping, from habitats disappearing in winter homes like the Amazon, to pet cats, to windows in skyscrapers. But the study suggests that neonicotinoids may be another reason. The European Union banned the insecticides in 2018; in the U.S., the Trump administration rolled back a partial ban and then, in May 2019, banned 12 products in a settlement with environmental groups. But 47 others are still in use. The products are also still widely used in other countries. The effects on bees and other pollinators were already concerning, Sutchbury says, but the impacts on birds raise even more concerns. “It’s really bad news that a vertebrate has been shown to be exposed to these things in the wild, and that when you do a controlled experiment, we can measure what effect it has,” she says. “And it’s quite dramatic.”

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About the author

Adele Peters is a staff writer at Fast Company who focuses on solutions to some of the world's largest problems, from climate change to homelessness. Previously, she worked with GOOD, BioLite, and the Sustainable Products and Solutions program at UC Berkeley, and contributed to the second edition of the bestselling book "Worldchanging: A User's Guide for the 21st Century."